Liars
Someone is lying in this conversation and it is hard to say who. It is either that, or this girlfriend is living in denial thinking that her man is loyal and would never cheat on her. We can not say for certain that he was cheating on her though, maybe he just wanted a night out with her and his friends.
Some couples can spend way too much time together and they need space every now and then. Perhaps he was just too afraid to tell his girlfriend because of how she would react. But we think the real reason is he just lied because he had some intention to get with another girl at some party.